Precise and effective fixed-pattern correction for logarithmic high dynamic range CMOS image sensors achieving low noise equivalent contrast over illumination

Abstract

Logarithmic High Dynamic Range CMOS (HDRC) image sensors exhibit the highest Dynamic Range and a straight forward image acquisition compared to other High Dynamic Range imagers or techniques. The nearly constant pixel random noise over the illumination range, in contrast to sensors with linear or piece-wise linear Opto Electronic Conversion Function (OECF), gives rise to a balanced contrast resolution. The Noise Equivalent Contrast (NEC) will be introduced as a figure of merit to compare both imager types with nonlinear and linear OECF, which leads to the incremental Signal-to-Noise Ratio (iSNR) and SNR, respectively. This will be shown by measurements of OECF and NEC performed with a logarithmic HDRC imager. The resulting iSNR, related to ISO15739, will be quantitatively compared to SNR data of a linear imager according to EMVA1288 standard. The benefits of the logarithmic imager come with the necessity to correct CMOS technology dependent pixel to pixel variations, namely the MOS transistor threshold and gain variations and the photodiode dark current distribution contributing to an overlaid Fixed Pattern in the raw image data. Depending on the required quality and the allowed computational complexity a Fixed Pattern Correction (FPC) algorithm should correct from the most dominant up to all three non-uniformity parameters per pixel in the digital signal chain of a camera.